Hi,
Im new to this forum but thought as Ive refered to it in the past and it has influenced my buying decisions Id let you have the benefit of my experience.
Ive referred to various forums over the last few months to try and get an idea of what the Sigma 150-500mm and the 120-400mm lenses are like. Ive now finally committed and bought the 120-400 and thought Id feedback my finding to those who may be interested. Theres a bit of a story behind this so Ill tell you that too. I live relatively near to Warehouse Expresss showroom so am in a position to go in and try the lenses before I buy. I initially went in to try the Tokina 80-400mm mainly because its light and small. I took a few photos with it in WHEs car park and bought them home to look at, the main thing that struck me was the appalling softness at f5.6, my Nikon 18-200 was actually sharper when I digitally zoomed by a factor of 2:1 to get the same equivalent length. The Tokina was somewhat better when stopped down a bit but even so I wasnt convinced.
Anyway, I then moved on to the Sigma 120-400mm and likewise took a few photos at different apertures in the car park and took the photos home to look at. The impression I got was pretty good, not as sharp at f5.6 as at other apertures but still not bad. So I went back and bought it. Got it home and thought Id have a more thorough look at the image quality. This is where it gets interesting, or at least I thought so. I set the lens and camera up, OS off, on my Manfrotto 055XPROB tripod with a 488RC ball head and took some photos of a ten pound note. I used available halogen light and took photos from f5.6 right through to f29, all with shutter speeds of 1 second and slower. When I looked at the images I was dismayed to see the lens was quite soft and was at its sharpest at f29, I was expecting optimum sharpness at around f11. So it wasnt looking good. I had, I should point out, made sure the lens I tested at the showroom was the same one I bought.
I had a think about it and thought maybe it was vibration that was causing the problem and that, ironically, the vibrations caused by the mirror movement were less significant on the longer shutter speeds used when at f29. So, I redid all the photos using a flash gun and the story was completely different. The lens was now, to my mind, pretty damn good at all apertures from f6.1 upwards and not bad at f5.6. Ive attached a composite image showing the same part of the £10 note taken from wide open to fully stopped down. Interestingly diffraction doesnt really seem to dominate until f29, I guess for the more mathematically minded that would allow you to work out the MTF50 value of the lens at wider apertures. The photos were taken on a D50 in RAW, converted using raw shooter essentials without any sharpening at any stage. Its worth pointing out that the f5.6 image does sharpen up very well using RSE, as Im sure youre all aware RAW sharpening is a lot better than doing it on jpegs as you dont tend to get the same level of artefacts. Its also worth pointing out that the OS seems to work pretty well too, Ive taken photos at 1/100th, 1/125th and 1/160th and seem to get about a 50% success rate at those sort of shutter speeds which I thought was pretty good. And its nowhere near as noisy as some people seem to think, maybe Im lucky and got a quieter one. And on a D50 or D90 it works fine in burst mode (continuous shooting), theres no delay between shots or anything untoward. So I guess reports of that are only the case on Canon bodies.
So in summary I reckon its a pretty good lens but Ive certainly learnt to be cautious when evaluating the quality of long lenses. There are many factors that can degrade image quality, focus and vibration being two obvious ones, and that mirror lock up isnt just a nicety when using a long lens on a tripod, its a necessity if you want optimum image quality, unfortunately not a feature the D50 has. Im sure most people reading this will think Im stating the obvious on various points but Ive certainly found it educational. I hope the attached images give anyone thinking of buying this lens an idea of the quality it can deliver.
OK I'm sorry I have to admit defeat, I can't see how I can add an image to this post, if anyone can let me know I'll be happy to attach the comparison photos. All 100% crops obviously.
I will hopefully receive it Thursday/Friday so will get some shots up asap to see if it is as good as some of the shots suggest (or not 